Just hours after Russ’s death at 3:21 a.m. on Aug. 31, EMS responders were called to a similar scene at Electric Zoo. This time, a 20-year-old named Olivia Rotondo, who allegedly uttered “I just took six hits of molly” minutes before a seizure that ushered in the end of her life. Their stories, which sent shivers through the all-you-need-is-love music-festival community, wouldn’t be the last. Just three weeks later, a 20-year-old Boston student and 19-year-old from the University of Virginia would suffer the same fate, both dying after ingesting MDMA, both while at a music venue.

In the aftermath, some focused on the potentially deadly contents of the so-called molly (which can contain anything from gasoline to baby powder); others pointed to the amount of the substance the young people consumed. “All Molly is bad Molly if you take six hits of it,” read the opening to a Gawker post about the deaths.

While the contents and amount of the MDMA they consumed most certainly played a role in their deaths, a new study this week highlights the importance of recognizing a third factor: the environment. Published in the Journal of Neuroscience, the study observed rats placed in a warm, active environment and analyzed their brain’s response to just moderate doses of molly.The results: Even moderate MDMA doses in conditions that mimic hot, crowded, social settings could be lethal to rats.

While a variety of studies have shown the potential of high doses of MDMA to spike body temperature, there had been a lack of significant research on moderate doses. Large doses of MDMA have been proven to increase body temperature, especially in warm environments, but there had been little research to prove that this is true with moderate doses as well. Without it, a dangerous assumption—in the face of a new concert season—remained: If concertgoers take pure MDMA, in moderation, they’ll have good clean fun.

With a thorough examination of exactly what happens during MDMA intoxication, it’s clear how damaging this belief can be.

A slang term for MDMA, molly is a purportedly pure form of ecstasy (3,4-methylenedioxy-N-methylamphetamine), a synthetic, psychoactive drug with amphetamine-like and hallucinogenic properties. Patented by Germany’s Merck and Co. in 1912, it increases the activity of these three neurotransmitters: serotonin, dopamine, and norepinephrine. Among the positive effects that result are euphoria, emotional warmth, heightened empathy, closeness to others, decreased anxiety, and a sense of well-being. Distortions in time and perception, which can last anywhere from three to six hours, make it particularly enticing to concertgoers. But the adverse effects of the drug are excruciating. Anxiety, restlessness, chills, sweating, blurred vision, and dehydration can occur with the use of MDMA. Those who recognize these symptoms early can prevent an overdose—but many do not. According to the federal Substance Abuse and Mental Health Services Administration, the number of emergency-room visits for under 21-year-olds with MDMA-related complications rose to more than 10,000 in 2011 from 4,460 in 2005.

Following this alarming rise, and four very public deaths, the study is an important part of a conversation that has only just begun.

“We know that high doses of MDMA can sharply increase body temperature to potentially lead to organ failure or even death,” National Institute on Drug Abuse Director Nora D. Volkow said in a statement. “However, this current study opens the possibility that even moderate doses could be deadly in certain conditions.”

In order to figure out how this is possible, the study’s authors had to figure out why it was happening in the first place. To do this, Eugene A. Kiyatkin, the study’s lead author and a scientist at the behavioral neuroscience branch of NIDA, zeroed in on the physiological mechanisms involved in fatal hyperthermia (overheating of the body to the point of death). “What’s new about this study is that it focuses on MDMA’s effects on brain temperature and clarifies the physiological mechanisms involved,” he tells The Daily Beast. “By recording data from the brain, muscle, and skin, we were able to show that MDMA raises heat in the brain by increasing brain activity as well as causing a strong, lasting constriction of peripheral blood vessels that prevents the normal dissipation of excess body heat to the external environment. These combined effects may be enhanced in conditions when brain and body heat production are naturally increased—such as social interaction—or when shedding excess heat is difficult, as it is in a warm environment.”

What the results proved is that the four who died after ingesting MDMA likely had no chance of making it in the first place. “We thought she was just dehydrated,” Dominique Vletter told Katie Couric in an October interview about the death of her 20-year-old University of Virginia roommate, Shelley Goldsmith. At a music festival in Washington, D.C. for Labor Day weekend last year, Vletter and her friends became worried when they learned Goldsmith had been taken away in an ambulance. “We were all starting to panic until someone said, ‘It’s OK. It’s only dehydration. Nothing could have happened, nothing’s wrong. This is all safe,’” Vletter recounts. By the next evening, Goldsmith was dead. “I still… I really don’t understand,” she says.

Dehydration, contrary to what the many reports following Goldsmith’s death said, was not one of the leading causes of death. Water couldn’t have saved Goldsmith—in fact, it may have made things worse.

When the body undergoes fatal hyperthermia—internal temperature rising to the point of brain damage—it’s fluid in the brain that leads the march to death. After MDMA raises heat production and constricts blood vessels, the internal body temperature increases, damaging brain cells irreparably. While this increase in brain temperature is incredibly destructive, it’s not the actual killer.

“The main mechanism leading to death from psychostimulant drugs appears to be leaking of the brain-blood barrier—which keeps most chemicals in the blood out of the brain—and water accumulation in the brain,” says Kiyatakin. It’s for this reason that concertgoers thinking hydration is the key to staying safe (“if you just drink enough water you’ll be fine”) is so dangerous. “The excessive use of liquids often used by people at dance clubs could be problematic if they take MDMA,” says Kiyatakin. “In addition to its other effects, it inhibits both sweating and urination, which expel water from the body. These factors could also contribute to dangerous water accumulation in the brain.”

Often used with other people in emotionally (and sometimes physically) intense situations, the conditions that induce fatal hyperthermia become more dangerous than the molly itself. “The only more dangerous scenario than a crowded music venue in which to take this drug would be during sex,” says Kiyatakin. “Sexual activity raises body heat and constricts peripheral blood vessels quite strongly, and these effects are also strongly affected by MDMA,” he says. “Therefore, combining MDMA with sex, especially in conditions where it is hard to shed excess heat, could be especially dangerous.”

Charles Grob, a professor of psychiatry and pediatrics at the UCLA School of Medicine and the researcher who conducted the first Phase-1 study of MDMA on humans in 1995, has closely followed the difference between recreational and therapeutic use of the drug. “I think the importance of this new research highlights that for a drug like MDMA, people’s reactivity is tied into where they’re taking it and under what conditions,” he says of this week’s study. “If it’s in a crowded club-like environment with warm temperatures and lots of people, plus some form of exercising like dancing… there’s a vulnerability to temperature regulation there that could be catastrophic.” Grob has witnessed the other side of MDMA, in which the drug is used in room-temperature, calm environments for therapy. “Taking it in a cool environment with access to fluids and no overcrowding, there’s far, far less risk of hyperthermia.”

Grob says there’s a dire need, especially ahead of music-festival season, to inform people about the risks associated with taking MDMA in this situation. “In the therapeutic model, it’s just an individual lying comfortably with a therapist where it’s a cool environment and lots of access to water,” he says of MDMA-therapy treatments, which have been successful in treating post-traumatic and other stress disorders. “But put someone in a crowded environment and all bets are off.” The pioneer in human MDMA research, Grob hopes that continued studies like this one will have the power to change the conversation about a drug that could, in the right circumstances, be useful for therapy.

For Kiyatkin and his colleagues, it’s about even more than informing the conversation. By looking at the drug’s effects in different conditions, he says scientists may soon be able to “define possible targets” for treating MDMA-induced brain hyperthermia. While there are a few treatments (such as intense body cooling in an ice bath or benzodiazepines) that can treat hyperthermia, once it reaches advanced stages, it’s nearly impossible to reverse.

It’s a fact that friends of Jeff Russ, Olivia Rotondo, Shelley Goldsmith, and Brittany Flannigan—the four who succumbed to molly overdoses at concerts last year—know all too well. This year, as confetti dances around the arms of those at music festivals across the nation, they’ll be remembering how fast a scene like this one can turn fatal.